This invention relates to improvements in an elevator speed control system for controlling a speed of an elevator car driven by an induction motor.
It is well known to control a firing angle of a thyristor device connected across an induction motor and a source of alternating current therefor thereby to control a rotational speed of the induction motor through a change in voltage applied across the motor. In this case, it has been generally practiced to apply an AC voltage controlled by the thyristor device across the induction motor in the power running mode of operation and to apply a DC voltage controlled by a separate thyristor device across the motor in the braking mode of operation.
There have been already proposed elevator control systems of the type utilizing the measure as above described to control a speed of an elevator car involved through the negative feedback control during the acceleration and deceleration of the car. Up to the deceleration of the car after the completion of the acceleration thereof, the travel of the car is effected by applying across an associated electric motor its rated voltage in order to decrease a quantity of heat generated and an electric power consumed by the motor.
The load on elevator systems can be of either a positive or a negative polarity as determined by a difference in weight between the particular elevator car and a counterweight therefor. The term "positive polarity" refers to the case an associated electric motor is required to produce a power running torque to bear a load involved while the term "negative polarity" refers to the case the electric motor should produce a braking torque to bear the load. For example, a load having a negative polarity can be caused when the elevator car is downwardly travelling with its full load. With the elevator car being accelerated under negative polarity loading the operation may be transferred to the braking mode midway of the acceleration. If, under these circumstances, the motor is applied with its rated voltage at the end of the acceleration then a large shock occurs when the operation changes from the braking mode to the rated voltage mode. This much injures a feeling of riding in an elevator car driven by the motor. In order to avoid this objection, there have been previously proposed elevator control systems of the type including means for gradually giving a command for the application of the rated voltage (which is called hereinafter a command for saturation) midway of the acceleration of the particular elevator car so as not to transfer the operation to the braking mode between the initiation of the acceleration of the car and the travel of the car at its rated speed even under negative polarity loading. However this type of elevator control systems has been disabled to effect the speed control upon the completion of the acceleration of the car because of the presence of a saturation signal originating from the command for saturation as above described. This has resulted in the disadvantage that a feeling of riding in the elevator car is made worse particularly under negative polarity loading because the feeling of riding in the elevator car upon the completion of the acceleration thereof is determined by the characteristics of the associated electric motor.